“…is it wise to put effort into developing a logline for an unresearched and undeveloped idea?…” IMO — Hell yeah. In fact, it’s half the point. In doing so you discover if the idea is actually worth pursuing or not, it can point you in the best direction, and an ‘undeveloped/ unresearched’ idea starts to develop.

The other half of the point is to develop it post completed screenplay (for hocking purposes…) — which can sometimes mean fitting a square peg into a round hole… in my (limited) experience starting out developing the logline before committing to a draft saves potentially months of work, handfuls of hair, and a dozen or so visits to a psychiatrist/ relationship counselor. Of course, you’re not married to the logline once into the draft — it just gives you a clear direction in which to head.

“…are writers sure the story behind the logline has a third act? Or even a second act?” — A good logline will give you the set-up of the world, hero and predicament (Act 1.) with an Inciting Incident (into Act 2) and should allude to the consequences and main challenge (Act 3) — of course, it should not reveal the conclusion — but the writer should certainly know how it ends.

“…can you actually write it?” — If the logline is solid you’ve got a clear statement on which you can base everything, from supporting plot points, character traits, reasons behind those character traits, other characters that will support the premise in your logine — in a nut shell — it can provide you with the direction to everything — of course — this is all just from my limited experience and research of other writers.